T7 phage particles have an icosahedral head, a short conical tail, six tail fibers, a head-tail connector, and an internal core (as well as 39,937 base pairs of double-stranded DNA). In hopes of generating a more symmetric particle for X-ray crystallography, tailless T7 phage mutants were created. These were examined in the STEM and also sent to Mario Cerritelli at NIH who discovered, and we verified, that the particles prefer to orient themselves with the connector down on the thin carbon substrate. These oriented particles are amenable to cryo-EM studies and he is analyzing the organization of the DNA in them. It also turned out that these tailless phage gave particularly accurate mass measurements in the STEM. There is now a series of tailless phage mutants containing full-length DNA, an 8% deletion of DNA, and a 16% deletion of DNA. These are being used in the STEM to refine precision mass mapping and are being analyzed by cryo-EM at NIH.